Cast of a Sahelanthropus tchadensis skull, a species of hominine that dates back to nearly 7 million years. Creative Commons

The results of a DNA test of a 400,000-year-old human thighbone are complicating what we thought we knew about human evolution. The bone, found in a cave in northern Spain known as the “Pit of Bones,” is the oldest human mitochondrial material ever sequenced. But rather than reveling in their discovery, scientists are getting frustrated.

The 400,000-year-old human thighbone is approximately 200,000 years older than modern humans. According to National Geographic, prior to the recent thighbone analysis, the oldest human DNA sequenced came from bones that were much younger, roughly 120,000 years old.

"Years ago, geneticists said they wouldn't be able to find DNA that was older than 60,000 years old," Jose Bermudez de Castro, a member of the team that excavated the “Pit of Bones” fossils, told the BBC. "Of course, that wasn't true. The techniques have advanced hugely." The details of the discovery were published Wednesday in the journal Nature.

Svante Paabo, the director of the Max Planck Institute for Evolutionary Antrhopolgy in Leipzig, Germany, added that the results “show that we can now study DNA from human ancestors that are hundreds of thousands of years old," adding that the discovery was “tremendously exciting."

The reason scientists are flustered? Researchers were expecting to uncover DNA from a Neanderthal, which once inhabited Europe and parts of Asia. But the DNA actually turned out to be very similar to a different kind of prehistoric people, the Denisovans, who lived thousands of miles away in what is now Siberia. First discovered in 2010, the Denisovans were a genetically distinct group of people who are known only by a pinkie bone and a tooth.

"The fact that they show a mitochondrial genome sequence similar to that of Denisovans is irritating," the lead author of the study, Matthias Meyer of the Max Planck Institute, told National Geographic. "Our results suggest that the evolutionary history of Neanderthals and Denisovans may be very complicated and possibly involved mixing between different archaic human groups."

The discovery challenges long-standing theories about human evolution, and could shed new light on how early species of human spread across different continents. According to The New York Times, scientists are even wondering if there couldn’t be several extinct human species populations that have yet to be found.

“Right now, we’ve basically generated a big question mark,” Meyer told The New York Times.